By Brandon Wronski, Special To Solar Power World
Various options exist for anchoring ground mounted solar arrays. These include drilled shaft piles (also called micropiles or caissons), driven piles and helical piers or ground screws. Racking manufacturers generally specify the depth, diameter and spacing of the anchors based on the site conditions including soil type and environmental factors i.e. wind, precipitation, etc.
The principal loads that must be supported are the vertical or compression loads from the weight of the panels, and the uplift or tension load caused by wind and ground movement thanks to the freezing and thawing of the ground (also called adfreeze force). Helical piles and micropiles work well in compression and tension applications and are ideally suited for solar panel installation. What are the differences between drilled shaft and helical piles? What equipment options are available for their installation?
Drilled shaft piles for solar array footings can vary anywhere from 6 to 24 inches in diameter and 5 to 30 feet deep, depending on site conditions and other variables. The drilled shaft or borehole is filled with high-strength cement grout or concrete. At times, steel casing or re-bar is used for reinforcement. Typically ‘straight’ shafts are drilled to the specified depth, but when necessary, a ‘belled’ shaft can be used where an underreaming tool expands the base of the shaft, which increases the base area and stability of the pile without increasing the shaft diameter.
The most efficient method for drilling the pile is determined by the depth required and ground conditions. Loose materials and overburden can be drilled effectively with augers. An auger bit is attached to the leading auger and cuts a hole slightly larger than the auger diameter which provides adequate clearance for the auger flights. Couplings at the end of each auger section (typically each auger section is 5 feet long) allow new lengths to be added quickly.
Standard continuous flight augers are used for regular open-hole drilling after which they are extracted and the borehole is filled with cement/grout. In less stable soil, hollow stem augers can be used to keep the hole open. The auger is then slowly extracted, removing the drill cuttings as concrete or grout is pumped through the hollow stem. Auger drilling requires sufficient rotary torque. Torque, in this case, is the turning force applied to the drill string. The denser the material being drilled, the larger the diameter of the bore and the deeper the hole is, the more torque required.
When drilling in rock formations, a DTH (down-the-hole) hammer is generally the most efficient method. The principals behind DTH hammers are similar to the way a hammer drill makes a hole in a brick wall. An air compressor is coupled to the drill rig to provide power for the hammer. This generates a rapid succession of impacts, causing the rock to break. The hammer and bit are connected to the drill head by hollow, threaded drill rods. These allow air to be flushed through the centre of the drill head, through the rods to the hammer and bit. The air also flushes the drill cuttings up and out of the bore hole. As the drilling progresses, additional rods are threaded together until the desired depth is reached.
Consolidated rock formations that require DTH hammer drilling are unlikely to collapse so casing is generally not required to keep the borehole open. But sometimes steel casing is used to provide the pile extra reinforcement. If required, casing can be twisted in by the drill head or attached to the DTH hammer and dragged down as the drilling progresses.
Finally, the drill string is extracted and the borehole is filled with cement/grout. Penetration rates of up to 2 to 3 feet per minute are possible with good conditions and suitable equipment. DTH hammer drilling requires less torque and down pressure than auger drilling as an air compressor is providing the force behind the hammer blows. The compressor must supply sufficient air pressure (PSI) and volume of air (CFM) which are both determined by the bore diameter, drill rod diameter and borehole depth.
Helical piles (also called helical piers) are basically steel shafts with helical flights that remain in the ground to which the above ground racking is fastened. Ground screws share the same basic principles as helical piles and are also used for anchoring solar arrays. Helical piles are twisted into the soil and require adequate rotary torque from the drill head, generally in the range of 4,000 to 10,000 ft-lbs. (5,000 to 13,000 Nm). In suitable soils and with proper drilling equipment, helical piles offer substantial benefits and can increase productivity considerably — but they are not viable in all ground conditions such as hard rock.
Helical piles do not require excavation or soil removal and there is minimal vibration disturbance. This can be very advantageous as it reduces the impact to the environment. The exclusion of concrete saves on curing time and allows for immediate loading once the pile is installed. It also means that the installation is not weather dependant. Helical piles can also be extracted for reuse or reconfiguration.
Another advantage is the elimination of drill cuttings from potentially contaminated soils and the ability to avoid grading and other ground disturbances, since maintaining the natural landscape of the site is often important to developers of alternative energy — especially in environmentally sensitive areas. Depending on ground conditions, helical piles can often be shorter in length and therefore cost less in installation time and energy consumption than comparable driven piles or drilled shafts. Some manufactures of helical piles for solar array anchoring assert installation rates as high as 500 piles per day.
What type of installation equipment is required for drilled shaft piles and helical piers? As we’ve discussed, adequate rotary torque is required for both auger drilling and helical pile installation. DTH hammer drilling through hard rock requires the proper tooling but less torque and down force. A drill rig with a high torque rotation head and a centre bore for air flushing offers the best of both worlds. As thousands of piles are often required, a compact, highly mobile rig is also vital.
“Straightness and accuracy are vital when drilling helical piles and boreholes for solar footings,” says Dan Carrocci of Determination Drilling in Ontario Canada. “A lot of valuable time can be lost setting up and maneuvering the rig for each hole.
“Here is where a rig with rubber tracks and lots of side to side and fore and aft mobility is key,” he adds.
Some drills have separate power units that can supply power up to 200 feet away, making the drill even more manoeuvrable. Such rigs are also ideal for limited access, or environmentally sensitive areas.
While solar fields often require the drilling of thousands of anchors, they are relatively shallow, and the drilling is reasonably straightforward. Highly efficient and cost effective drilling equipment and technology is available which can lower installation costs, which improves the industry and increases global sustainability.
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